Abstract: The security reversible key with an assigned cylinder Z has a blocking groove BN with a coded blocking depth (B1, B2, B3), which runs parallel to the axis of the key (x) from the tip of the key to at least the first position (P1) of a row of tumbler pins (A2) on the key. In the assigned cylinder at least at the rearmost coding position (P1) a pair of tumbler pins corresponding to the blocking groove BN with a blocking tumbler pin BZ and an extended blocking counter pin BG are foreseen, whereby the blocking counter pin BG impinges on the cylinder housing (10), if the blocking groove is insufficiently deep and with this the complete insertion of a key with an insufficiently deep blocking groove is blocked by the pair of blocking tumbler pins. Simultaneously the blocking tumbler pin BZ with the counter pin BG at the position (P1) also serves as coding tumbler pin.